Process for preparing low-acid-value phosphoric esters

ABSTRACT

The process for producing a low acid value phosphoric ester of the present invention is characterized by comprising treating a phosphoric ester having an acid value with an organic ortho-acid ester. The phosphoric ester preferably has a specific structure. The organic ortho-acid ester is preferably an ester of orthoformic acid, orthoacetic acid or orthopropionic acid with an alkyl group having 1 to 4 carbon atoms.

This application is a 371 of PCT/JP00/03693 filed Jun. 7, 2000.

TECHNICAL FIELD

The present invention relates to a process for producing a phosphoricester of low acid value. More particularly, it relates to a process forobtaining a phosphoric ester having a low acid value and excellentphysical properties, such as heat resistance, storage stability, andhydrolysis resistance. The phosphoric ester obtained is useful as aplasticizer or a flame retardant for synthetic resins.

BACKGROUND ART

It is known that phosphoric esters are synthesized by processescomprising dehydrochlorination reaction between phosphorus oxychlorideand an alcohol or a phenol. Because these processes do not accomplishperfect esterification, however, the resulting phosphoric esters have anacid value ascribed to a phosphoric acid radical or an acid chloridederived from the starting material. Phosphoric esters having an acidvalue are not satisfactory in heat resistance, hydrolysis resistance,and storage stability. When exposed to high temperature, they undergoconsiderable coloration and, when added to a resin as a flame retardant,invite reductions of the physical properties of the resin and colorationof the resin. Phosphoric esters having an acid value have anotherproblem that they corrode a mold for molding the resin. To avoid theseproblems, it is desirable to control the acid value of phosphoric estersto 1.0 (mg/KOH) or smaller.

In order to obtain a phosphoric ester of low acid value, it has been agenerally followed practice that a phosphoric ester is purified byneutralization with a basic substance, for example, in a wet processusing an alkali metal hydroxide such as sodium hydroxide or in a dryprocess using an alkali metal compound such as calcium carbonate ormagnesium hydroxide, followed by washing with water or distillation.

However, where a phosphoric ester having a high viscosity is purified,the wet neutralization with an alkali metal hydroxide meets difficultyin separation between an aqueous layer and an oily layer, which not onlyneeds a long process time but results in incorporation of a trace amount(e.g., several ppm to several hundreds of ppm) of the alkali metal intothe oily layer separated. If any alkali metal remains in thepurification step of phosphoric esters, it adversely affects the heatresistance and hydrolysis resistance of the phosphoric esters and causesreductions of physical properties of some kinds of resins, such as achange in composition.

For the purpose of reducing the residual alkali metal content, aphosphoric ester may be diluted with an organic solvent to reduce theviscosity or subjected to salting out to improve separation into anaqueous layer and an oily layer. Nevertheless these manipulations stillallow a trace amount of the alkali metal to remain in the product layer.Therefore, removal of the alkali metal is usually conducted by washingseveral times. The same problem arises in the case of dryneutralization.

Some phosphoric esters are not suitable to be wet neutralized withalkali metals because they get emulsified on wet neutralization andcannot be separated into an aqueous layer and a product layer.

Purification of phosphoric esters by distillation is also adopted.Purification by distillation is freed of the above-described problem ofalkali metal remaining with respect to phosphoric esters having a lowmolecular weight but requires a distillation apparatus having a highfractionation effect, such as a rectifier, in order to remove impuritiesother than alkali metals which will reduce the physical properties ofphosphoric esters (for example, heat resistance, storage stability andhydrolysis resistance). There is another problem that distillationpurification is more difficult with a phosphoric ester having a largermolecular weight. Further, distillation purification has a poor yield,resulting in an increased cost of the phosphoric ester.

Japanese Patent Laid-Open No. 67685/96 discloses a process of producinga phosphoric ester, which comprises treating a crude phosphoric esterwith an epoxy compound, followed by heat treatment in the presence ofwater. However, this process involves heat treatment in the presence ofwater (hereinafter “water/heat treatment”) after epoxy treatment, whichmakes the steps complicated. Further, should the water/heat treatment beinsufficient, the product is deteriorated in both acid value and hue.Even when the water/heat treatment is sufficient, the product isunsatisfactory in hue and storage stability.

Accordingly, an object of the present invention is to provide a processfor producing a phosphoric ester having a low acid value and excellentin heat resistance, hydrolysis resistance and storage stability.

DISCLOSURE OF THE INVENTION

The above object of the invention is accomplished by a process forproducing a low acid value phosphoric ester characterized by treating aphosphoric ester having an acid value with an organic ortho-acid ester.

BEST MODE FOR CARRYING OUT THE INVENTION

The process of producing a low acid value phosphoric ester according tothe present invention will be described in detail.

The phosphoric esters which can be treated by the process of the presentinvention are known in the art usually for use as plasticizers and/orflame retardants for resins. As far as they have such acid values ashave to be reduced, the kinds, the process of making and the like arenot particularly limited. Typical examples of the phosphoric esters usedin the present invention are those represented by general formula (1):

wherein R₁, R₂, R₄, and R₅, which may be the same or different, eachrepresent an alkyl group having 1 to 10 carbon atoms or an aromaticgroup represented by general formula (2) shown below; R₃ represents adivalent aromatic group represented by general formula (3) or (4) shownbelow; and n represents 0 to 30.

wherein A₁ and A₂ each independently represent a hydrogen atom or analkyl group having 1 to 10 carbon atoms.

wherein A₃, A₄, A₅, A₆, A₇, and A₈ each independently represent ahydrogen atom, an alkyl group having 1 to 4 carbon atoms, a cycloalkylgroup, an aryl group, an alkoxy group, a nitro group, a halogen atom ora cyano group; and B represents a single bond, divalent S, a sulfonegroup, or an alkylidene or alkylene group having 1 to 5 carbon atoms.

In general formulae (1) and (2), the alkyl group as represented by R₁,R₂, R₄, R₅, A₁, and A₂ includes methyl, ethyl, propyl, isopropyl, butyl,isobutyl, sec-butyl, tert-butyl, amyl, tert-amyl, hexyl, 2-ethylhexyl,n-octyl, nonyl, and decyl. The alkyl group having 1 to 4 carbon atoms asrepresented by A₃, A₄, A₅, A₆, A₇, and A₈ includes methyl, ethyl,propyl, butyl, isobutyl, sec-butyl, and tert-butyl. The cycloalkyl groupincludes cyclohexyl. The aryl group includes phenyl, cresyl, xylyl,2,6-xylyl, 2,4,6-trimethylphenyl, butylphenyl, and nonylphenyl. Thealkoxy group includes methoxy, ethoxy, propoxy, and butoxy. The halogenatom includes a fluorine atom, a chlorine atom, and a bromine atom. Thegroup represented by general formula (2) includes phenyl, cresyl, xylyl,2,6-xylyl, butylphenyl, and nonylphenyl. The alkylidene group having 1to 5 carbon atoms as represented by B includes ethylidene and2,2′-propylidene. The alkylene group includes methylene, ethylene,trimethylene, and tetramethylene.

The acid value of a phosphoric ester to which the process of the presentinvention is applied includes one attributed to the acid radicalremaining after the synthesis of the phosphoric ester and one attributedto the acid radical produced during storage.

The above-described phosphoric esters which can be used in the processof the present invention can be obtained by methods known in the art.They are usually obtained by allowing phosphorus oxychloride to reactwith an appropriate alcohol or phenol in the absence or presence of acatalyst such as a Lewis acid (e.g., aluminum chloride, magnesiumchloride or titanium tetrachloride). Specifically, a phosphoric triesteris produced by allowing phosphorus oxychloride to react with a phenol inthe presence of a Lewis acid catalyst (see, e.g., G. Jacobsen, Ber.81519 (1875) and M. Rapp, Ann. 224 156 (1884)). An aromatic bisphosphateis obtained by allowing phosphorus oxychloride to react with an aromaticmonohydroxy compound (a monohydric phenol) in the presence of a Lewisacid catalyst and allowing the resulting diarylphosphorohalidate toreact with an aromatic dihydroxy compound (a dihydric phenol) in thepresence of the same catalyst (see, e.g., Japanese Patent. Laid-Open No.1079/93). An aromatic diphosphate is also obtainable by allowingphosphorus oxychloride to react with a dihydroxy compound, removing theunreacted phosphorus oxychloride, and allowing the product to react withan aromatic monohydroxy compound (see Japanese Patent Laid-Open No.227632/88). An aromatic diphosphate is also obtainable by allowingphosphorus oxychloride to react with a mixture of a monohydroxy compoundand a dihydroxy compound.

Preferred examples of the alcohols which can be used for the productionof the phosphoric esters include aliphatic alcohols, such as methanol,ethanol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, octylalcohol, 2-ethylhexyl alcohol, capryl alcohol, nonyl alcohol, andn-decyl alcohol; alicyclic alcohols, such as cyclohexanol; and aromaticalcohols, such as benzyl alcohol.

Preferred examples of the phenols which can be used in the production ofthe phosphoric esters include phenol, cresol, xylenol, resorcin,hydroquinone, bisphenol A, tetrabisphenol A, bisphenol F, bisphenol S,biphenol, and naphthol.

The reaction conditions for preparing the phosphoric esters, such as theamount of the reaction catalyst, the reacting ratio of phosphoric acidand the alcohol or phenol, the reacting ratio of phosphorus oxychlorideand the alcohol or phenol, the reaction temperature, and the reactiontime, can be decided appropriately within known ranges.

Specific examples of the phosphoric esters of general formula (1) inwhich n is 0 are triphenyl phosphate, tricresyl phosphate,diphenyl-2-ethylcresyl phosphate, tri(isopropylphenyl) phosphate,methyldiphenyl phosphate, phenyldiethyl phosphate, diphenylcresylphosphate, and tributyl phosphate. Those in which n is 1 or greaterinclude phenyl.bisphenolA.polyphosphate,cresyl.bisphenoIA.polyphosphate, phenyl.cresyl.bisphenolA.polyphosphate,xylyl.bisphenolA.polyphosphate,phenyl-p-t-butylphenyl.bisphenolA.polyphosphate,phenyl.isopropylphenyl-bisphenolA.polyphosphate,phenyl.resorcin.polyphosphate, cresyl.resorcin.polyphosphate,phenyl.cresyl.resorcin.polyphosphate, xylyl.resorcin.polyphosphate,phenyl-p-t-butylphenyl-resorcin-polyphosphate, andphenyl.isopropylphenyl.resorcin.polyphosphate.

While the production process of the present invention is applicable toany of the above-described phosphoric esters, particularly pronouncedeffects are produced in application to the phosphoric esters of generalformula: (1) in which n is 1 or greater, which have large molecularweights and are difficult to purify by general treating methods.

Of the phosphoric esters in which n is I or greater, (a) those ofgeneral formula (1) wherein R₁, R₂, R₄, and R₅ are each the grouprepresented by general formula (2), R₃ is the group represented bygeneral formula (4), and B is 2,2′-propylidene; especially (a′) thosewherein A₁ and A₂ in the group represented by general formula (2) andA₅, A₆, A₇, and A₈ in the group represented by general formula (4) areeach a hydrogen atom; and (b) those of general formula (1) wherein R₁,R₂, R₄, and R₅ are each the group represented by general formula (2),and R₃ is the group represented by general formula (3) enjoyparticularly outstanding effects when the production process of thepresent invention is applied thereto.

The phosphoric esters thus produced usually contain considerableimpurities. They are subjected to acid treatment with hydrochloric acid,etc. and refluxing dehydration to remove the impurities. The productionprocess of the present invention is for effective reduction of the acidvalue of the thus obtained phosphoric esters.

The above-mentioned phosphoric esters include solid ones and liquidones. While the production process of the present invention isapplicable to both of them, it is preferred that a crude phosphoricester which is solid be used as dissolved in a solvent. Any solvent canbe used here as long as it is capable of dissolving the crude phosphoricester and does not interfere with the action of an organic ortho-acidester that is assumed to be as described later. Specific examples of thesolvents include aromatic solvents, such as toluene, xylene, anddichlorobenzene, aliphatic solvents, such as hexane and heptane, andalicyclic solvents, such as cyclohexane. Preferred of them are aromaticsolvents because of their satisfactory capability of dissolving crudephosphoric esters. In the present invention, it is unfavorable to use ahydroxyl-containing alcoholic compound or an amino-containing compoundas a solvent for dissolving a crude phosphoric ester. The reason isthat, for one thing, these compounds, while being heat treated, undergointeresterification with a phosphoric ester to be purified to reduce thepurity or reaction with impurities to form salts and, for another, theyare likely to react with the organic ortho-acid ester.

In the production process of the present invention, the phosphoric esterhaving an acid value is treated with an organic ortho-acid ester. Thetreatment with an organic ortho-acid ester according to the presentinvention is to esterify the acid component in the impurities present inthe phosphoric ester having an acid value with the organic ortho-acidester.

Examples of preferred organic ortho-acid esters include alkyl esters andaromatic esters of organic ortho-acids, such as orthoformic acid,orthoacetic acid, and orthopropionic acid. Specific examples aretrimethyl orthoformate, triethyl orthoformate, tributyl orthoformate,trimethyl orthoacetate, triethyl orthoacetate, tributyl orthoacetate,trimethyl orthopropionate, triethyl orthopropionate, tributylorthopropionate, and triphenyl orthoformate.

Particularly preferred of them are esters of orthoformic acid,orthoacetic acid or orthopropionic acid with an alkyl group having 1 to4 carbon atoms because they have a small molecular weight and, therefor,an excess of them can easily be removed by distillation.

While any of the above-described organic ortho-acid esters can be usedin the present invention, it is economically preferred to use organicortho-acid esters that are liquid at ambient temperature. Examples oforganic ortho-acid esters that are liquid at ambient temperature aretrimethyl orthoformate, triethyl orthoformate, tributyl orthoformate,trimethyl orthoacetate, triethyl orthoacetate, tributyl orthoacetate,trimethyl orthopropionate, triethyl orthopropionate, and tributylorthopropionate. It is still preferred to use trimethyl orthoformate,triethyl orthoformate, trimethyl orthoacetate or triethyl orthoacetate,which have a molecular weight of from 100 to 500. Excess of an organicortho-acid ester having a large molecular weight is difficult to removeby distillation. When an organic ortho-acid ester having a smallmolecular weight is used, excess can easily be removed by distillation.

The manner of treating the phosphoric ester having an acid value withthe organic ortho-acid ester is not particularly restricted andappropriately selected according to the physical properties orreactivity, etc. of the organic ortho-acid ester used. For example, whenthe phosphoric ester having an acid value is treated with liquidtriethyl orthoformate, the treatment can be carried out by adding theorganic ortho-acid triethyl ester to the phosphoric ester followed byheating.

The temperature for treating the phosphoric ester having an acid valuewith the organic ortho-acid ester is decided appropriately depending onthe kind of the organic ortho-acid ester used. In using triethylorthoformate, for example, a preferred temperature is 50° C. to 160° C.,particularly 80° C. to 140° C., with the reactivity and a volatilizationloss of triethyl orthoformate taken into consideration. At reactiontemperatures lower than 50° C., it is likely that the organic ortho-acidester exhibits insufficient reactivity and needs an extended reactiontime and that an insufficient reaction results in a failure tocompletely remove impurities after the treatment of the phosphoric esterwith the organic ortho-acid ester. On the other hand, treatingtemperatures above 160° C. exceed the boiling point of the organicortho-acid ester, resulting in reduction of the reaction efficiency.

The time for treating the phosphoric ester having an acid value with theorganic ortho-acid ester is decided appropriately according to the kindand the molecular weight of the organic ortho-acid ester used and thereaction temperature. In general, the treating time is preferably about10 minutes to 3 hours. In using, for example, triethyl orthoformatehaving a molecular weight of about 148, a preferred treating tithe is 1to 2 hours.

In treating the phosphoric ester having an acid value with the organicortho-acid ester, it is theoretically sufficient for the organicortho-acid ester to be added to the phosphoric ester in an amountcorresponding to the acid value of the phosphoric ester. It is preferredhowever that the organic ortho-acid ester be added in slight excess overthe amount corresponding to the acid value of the phosphoric ester fromthe considerations for the reactivity of the organic ortho-acid ester,the residual water content in the phosphoric ester, and a volatilizationloss of the organic ortho-acid ester in case where it has a low boilingpoint. In general, the ratio of the phosphoric ester to the organicortho-acid ester added is preferably about 1:1 to about 1:20 by molebased on the acid value of the crude phosphoric ester.

While not having been confirmed, it is assumed that the followingreaction takes place while a phosphoric ester having an acid value istreated with an organic ortho-acid ester according to the process of thepresent invention. It is considered that the acid components present inthe phosphoric ester having an acid value are, esterified with theorganic ortho-acid ester in the treatment, whereby the acid value of thephosphoric ester seems to be reduced.

After the phosphoric ester having an acid value is treated with theorganic ortho-acid ester as described above, it is preferred, ifnecessary, that the treated liquid be washed with water. By washing withwater, the unreacted organic ortho-acid ester is hydrolyzed and removed.

Where washing with water is conducted, a single washing operation wouldbe enough. The amount of water for washing is adequately about 10 to200% by weight based on the total weight of the reaction mixture.Specifically, the washing operation is carried out by, for example,adding water to the treated liquid resulting from the treatment of thecrude phosphoric ester with the organic ortho-acid: ester, mixing bystirring, and allowing the mixture to stand to let the mixture separateinto an aqueous layer and an oily layer, and separating and removing theupper aqueous layer by means of a separatory funnel, etc.

After washing with water, the residual water is removed to give apurified phosphoric ester. The residual water can be removed by methodsgenerally employed in the art but preferably by distillation underreduced pressure. The distillation temperature is preferably 80 to 160°C.

Where the crude phosphoric ester is solid, the solvent that has beenused for dissolving it may be removed simultaneously with thedistillation under reduced pressure. The solvent can also be removed bydehydration by drying followed by steam distillation.

The thus purified phosphoric ester contains substantially no impurities,has a low acid value, and is excellent in physical properties such asheat resistance, storage stability and hydrolysis resistance.

Having a low acid value, the phosphoric esters purified by the processof the present invention, when used as a plasticizer or a flameretardant for resins, neither cause reduction of physical properties ofthe resins nor corrode a metallic mold used for molding the resins.Having excellent heat resistance, they undergo no compositional changeat molding temperatures and no coloration. Therefore, they are suitablyused as a plasticizer or a flame retardant for resins.

The present invention will now be illustrated in greater detail by wayof Examples, but the present invention is not construed as being limitedthereto.

Synthesis Example 1 Synthesis of Compound A (Crude Phosphoric Ester)

Phosphorus oxychloride (613 g) and bisphenol A (228 g) were allowed toreact under atmospheric pressure in the presence of magnesium chlorideas a catalyst. After excess of phosphorus oxychloride was removed byevaporation, phenol (376 g) was added to the residue, and the reactionwas further continued under reduced pressure until a theoretical amountof hydrochloric acid was generated. The reaction mixture was dilutedwith xylene and washed with a hydrochloric acid aqueous solution toremove the catalyst. The reaction mixture was refluxed under reducedpressure to remove the water content to give compound A (658 g). Theresulting composition was a pale yellow viscous liquid having an acidvalue of 1.02.

Synthesis Example 2 Synthesis of Compound B (Crude Phosphoric Ester)

Phosphorus oxychloride (460 g) and resorcin (110 g) were allowed toreact under atmospheric pressure in the presence of aluminum chloride asa catalyst. After excess of phosphorus oxychloride was removed byevaporation, phenol (329 g) was added to the residue, and the reactionwas further continued under reduced pressure until a theoretical amountof hydrochloric acid was generated. The reaction mixture was dilutedwith xylene and washed with a hydrochloric acid aqueous solution toremove the catalyst. The reaction mixture was refluxed under reducedpressure to remove the water content to give compound B (517 g). Theresulting composition was a pale yellow viscous liquid having an acidvalue of 1.96.

EXAMPLE 1

Triethyl orthoformate (7.1 g) was added to compound A (658 g) obtainedin Synthesis Example 1, and the mixture was allowed to react at 120° C.for 2 hours. After the temperature dropped to 80° C., the reactionmixture was washed with 200 g of water. The oily layer was distilledunder reduced pressure to remove the water content to obtain purifiedproduct A, which was a pale yellow viscous liquid having an acid valueof 0.04.

EXAMPLE 2

Triethyl orthoacetate (7.8 g) was added to compound A (658 g) obtainedin Synthesis Example 1, and the mixture was allowed to react at 120° C.for 2 hours. After the temperature dropped to 80° C., the reactionmixture was washed with 200 g of water. The oily layer was distilledunder reduced pressure to remove the water content to obtain purifiedproduct B, which was a pale yellow viscous liquid having an acid valueof 0.03.

EXAMPLES 3 AND 4

The same procedures of Example 1 were carried out, except that anorganic ortho-acid ester was allowed to react under the temperature andtime conditions shown in Table 1, to obtain purified products C and D,which were pale yellow viscous liquids having an acid value of 0.08 and0.03, respectively.

EXAMPLE 5

Triethyl orthoformate (4.4 g) was added to compound B (517 g) obtainedin Synthesis Example 2, and the mixture was allowed to react at 120° C.for 2 hours. After the temperature dropped to 80° C., the reactionmixture was washed with 200 g of water. The oily layer was distilledunder reduced pressure to remove the water content to obtain purifiedproduct E, which was a pale yellow viscous liquid having an acid valueof 0.06.

EXAMPLE 6

The same procedures of Example 5 were carried out, except for increasingthe amount (6.6 g) of triethyl orthoformate to 1.5 times as much as thatused in Example 5 and changing the reaction time to 1 hour, to obtainpurified product F, which was a pale yellow viscous liquid having anacid value of 0.06.

Comparative Example 1

The compound obtained in Synthesis Example 1 was distilled under reducedpressure to remove the solvent. The resulting product was designatedpurified product G. It was a pale yellow viscous liquid having an acidvalue of 1.02.

Comparative Example 2

Propylene oxide (7.1 g) was added to compound A (658 g) obtained inSynthesis Example 1, and the mixture was allowed to react at 1 20° C.for 2 hours. After the temperature dropped to 80° C., the reactionmixture was washed with 200 g of water and then heat treated at 140° C.for 30 minutes. The reaction mixture was distilled under reducedpressure to remove the water content to obtain purified product H, whichwas a pale yellow viscous liquid having an acid value of 0.08.

Comparative Example 3

The procedures of Comparative Example 2 were repeated, except that theheat treatment of Comparative Example 2 was not conducted, to obtainpurified product I. Purified product I was a pale yellow viscous liquidhaving an acid value of 0.09.

In Table 1 are shown the physical property (acid value) of the purifiedproducts obtained in Examples 1 to 6 and Comparative Examples 1 to 3,the acid value after a heat resistance test at 80° C. for 14 days, andthe coloration (hue) after a heat resistance test at 250° C. for 3hours. The heat resistance test was carried out by setting a desiccatorcontaining water in a thermostat.

TABLE 1 Organic Reaction Acid Value Heat Resistance Test Ortho-acidEster Temp. Reaction Heat after Acid Value Hue (Gardner Scale) or EpoxyCompound (° C.) Time (hr) Treatment Purification (80° C. × 14 dys) (250°C. × 3 hrs) Example 1 triethyl orthoformate 120 2.0 un-done 0.04 0.3 4Example 2 triethyl orthoacetate 120 2.0 un-done 0.03 0.4 4 Example 3triethyl orthoformate  90 3.0 un-done 0.09 0.3 4 Example 4 triethylorthoformate 140 1.0 un-done 0.03 0.3 4 Example 5 triethyl orthoformate120 2.0 un-done 0.06 0.8 5 Example 6 triethyl orthoformate 120 1.0un-done 0.06 0.7 5 Comp. none — — un-done 1.02 2.0 6 Example 1 Comp.propylene oxide 120 2.0 done 0.08 1.0 8 Example 2 Comp. propylene oxide120 2.0 un-done 0.09 1.8 11  Example 3

It is apparent from the results of Table 1 that the phosphoric estersproduced by the process of the present invention have a lower acid valueand show a reduced rate of increase of acid value in the heat resistancetest than those produced by the process of Comparative Examples(conventional production processes) and are therefore excellent instorage stability. Further, they undergo less coloration in the heatresistance test as compared with the comparative phosphoric esters.Thus, they are suit ably used as a plasticizer or a flame retardant forresins. It was additionally confirmed that the process of the presentinvention is simpler, involving no water/heat treatment.

Industrial Applicability

According to the production process of the present invention, highpurity phosphoric. esters having a low acid value and excellent inphysical properties such as heat resistance, hydrolysis resistance andstorage stability can be obtained from crude phosphoric esters throughsimple operations.

What is claimed is:
 1. A process for producing a purified phosphoricester having a low acid value, which comprises: treating a crudephosphoric ester having an acid value with an organic ortho-acid ester,wherein said crude phosphoric ester is a phosphoric ester represented byformula (1):

wherein R₁, R₂, R₄, and R₅, which may be the same or different, eachrepresent an alkyl group having 1 to 10 carbon atoms or an aromaticgroup represented by formula (2) shown below; R₃ represents a divalentaromatic group represented by formula (3) or (4) shown below; and nrepresents 0 to 30

wherein A₁ and A₂ each independently represent a hydrogen atom or analkyl group having 1 to 10 carbon atoms

wherein A₃, A₄, A₅, A₆, A₇, and A₈ each independently represent ahydrogen atom, an alkyl group having 1 to 4 carbon atoms, a cycloalkylgroup, an aryl group, an alkoxy group, a nitro group, a halogen atom ora cyano group; and B represents a single bond, divalent S, a sulfonegroup, or an alkylidine or alkylene group having 1 to 5 carbon atoms. 2.The process for producing a purified phosphoric ester having a low acidvalue according to claim 1, wherein n in formula (1) is 1 or greater. 3.The process for producing a purified phosphoric ester having a low acidvalue according to claim 2, wherein, in formula (1) , R₁, R₂, R₄, and R₅are each the group represented by formula (2), R₃ is the grouprepresented by formula (4), and B is 2,2′-propylidene.
 4. The processfor producing a purified phosphoric ester having a low acid valueaccording to claim 3, wherein A₁ and A₂ in the group represented byformula (2) and A₅, A₆, A₇, and A₈ in the group represented by formula(4) are each a hydrogen atom.
 5. The process for producing a purifiedphosphoric ester having a low acid value according to claim 2, wherein,in formula (1) , R₁, R₂, R₄, and R₅ are each the group represented byformula (2), and R₃ is the group represented by formula (3).
 6. Theprocess for producing a purified phosphoric ester having a low acidvalue according to claim 1, wherein said organic ortho-acid ester is anester of orthoformic acid, orthoacetic acid or orthopropionic acid withan alkyl group having 1 to 4 carbon atoms.
 7. The process for producinga purified phosphoric ester having a low acid value according to claim1, wherein said organic ortho-acid ester is used in an amount of 1 to 20mol per acid value of the crude phosphoric ester.
 8. The process forproducing a purified phosphoric ester having a low acid value accordingto claim 1, wherein the treating is carried out at a temperature rangingfrom 50° C. to 160° C.
 9. The process for producing a purifiedphosphoric ester having a low acid value according to claim 1, whereinthe treating is carried out for a period of time ranging from 10 minutesto 3 hours.
 10. The process for producing a purified phosphoric esterhaving a low acid value according to claim 1, wherein the treating iscarried out at a ratio of crude phosphoric ester to organic ortho-acidester ranging from 1:1 to 1:20 by mole based on the acid value of thecrude phosphoric ester.